Is this a client your agency has never been involved with? Or one that you personally just haven't been involved with?

If the agency has said client, you should password protect your work. The agency can't prove that they never asked you to work on that client or that that client didn't have said idea for their product, so you might be violating your NDA.

Now that being said, most clients do not look at people's books and even if they did, them taking legal action is probably more work than they want to do. But still you could cause problems between you and your agency or ex-agency.

Personally, I wouldn't but that's something you might want to consult a lawyer on. I think at the very minimum you should make it abundantly clear that your shop has no affiliation with Coke/Sony otherwise it could look like those companies are implicitly endorsing your work.

The other issue is you'd be showing spec work on a company website. If I saw that, I'd assume you don't have enough real world experience and are trying to pad your work. If you want more ways to showcase your talents, either work on branding for your own company or find a charity and offer them free design work.

Show it. Just state clearly it is a concept. No need for much details about was it ordered or not.

In digital design people do huge number of Nike, Facebook and any other famous brand/app/site concepts all the time and publish in Behance or Dribble.
Also huge companies like Sony and CC they can never keep track what was ordered and what not. In many cases some mid level project managers even order something just to present for their seniors and get an basic ideas approved. Different divisions, countries etc. Some cases re-sellers order their own marketing materials etc. It is just important that you don't lie. State it is concept and show what you got. I imagine that you are just starting when you want to publish something like this and hopefully it is something great and you can get attention of some bigger agencies. And who knows maybe working for Sony is closer then you can imagine.